1. Field of the Invention
The present invention relates generally to spacecraft attitude control systems.
2. Description of the Related Art
Spacecraft attitude control is essential because spacecraft must generally be properly oriented to perform their intended service. Communication spacecraft, for example, typically provide communication services for service areas. Accordingly, antenna systems of these spacecraft generate payload beams that form payload footprints on the earth and it is critically important to reduce service error which is any difference between a payload footprint and its respective service area. This can only be accomplished by precise control of a spacecraft""s attitude.
In an exemplary star sensor-based attitude determination and control system, a spacecraft""s attitude is sensed with star sensors of a stellar attitude determination system and appropriately altered with torque generators that are coupled to the spacecraft""s body. Star sensors are complex semiconductor systems which generally include a) an array of light sensitive elements that collect charge in response to incident star light, b) an arrangement of charge-transfer elements that readout the collected charges and c) an output structure that converts the transferred charges to corresponding star-sensor signals.
The collected charges of the array are generally processed into star centroids and each transfer of the processed charges from the star sensor is typically referred to as a data frame. Star-sensor signals are thus provided at a frame rate and denote vertical and horizontal coordinates of stars in the star-sensor""s field-of-view. Preferably, the star-sensor signals also denote the magnitudes of these stars.
An estimate of the spacecraft""s attitude at the time of a data frame can be formed by identifying the stars that generated the star-sensor signals of that data frame. Identification is generally realized by matching the star-sensor signals to a known set of stars and the known set is typically accessed from stored star catalogs. However, star-sensor fields-of-view are limited (e.g., to horizontal and vertical ranges of 8xc2x0) so that attitude determination systems sometimes fail to provide sufficient star-sensor signals to enable an identification. The probability of such failure is increased when the intensity of the star-sensor signals is reduced by maneuvers (e.g., those during transfer orbits) that rotate the spacecraft at a rate that denies sufficient time for the star-sensor light sensitive elements to fully charge in response to incident star light.
The present invention is directed to attitude acquisition methods and systems which reduce the time generally required to acquire spacecraft attitude estimates and enhance the probability of realizing such estimates.
These goals are realized with methods and systems that, over a time span xcex94t, receive successive frames of star-sensor signals that correspond to successive stellar fields-of-view, estimate spacecraft rotation xcex94r throughout at least a portion of the time span xcex94t, and, in response to the spacecraft rotation xcex94r, process the star-sensor signals into a processed set of star-sensor signals that denote star positions across an expanded field-of-view that exceeds any of the successive fields-of-view. The expanded field-of-view facilitates identification of the stars that generated the processed set of star-sensor signals to thereby acquire an initial attitude estimate.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.